A wavelength tunable optical transmitter apparatus relates to the field of communications technologies and includes a plurality of multi-longitudinal mode lasers (1), a cyclic wavelength demultiplexer/multiplexer (2), and a reflector (3). The multi-longitudinal mode lasers output multi-longitudinal mode signals having a periodically repeated frequency interval, where the period is Δfmode. The cyclic wavelength demultiplexer/multiplexer performs periodically repeated filtering on the multi-longitudinal mode signals input by the multi-longitudinal mode lasers, to obtain single frequency signals, where a repetition period of a filter window is Δfband, Δfmode is different from Δfband, and Δfmode and Δfband are not in an integer-multiple relationship, and then, multiplexes the plurality of single frequency signals and outputs the multiplexed signal. The reflector reflects the multiplexed signal. The reflected multiplexed signal separately returns to the multi-longitudinal mode laser by using the cyclic wavelength demultiplexer/multiplexer again.